This invention relates to a rotary drill bit of the so-called sealed bearing roller cutter type used to drill oil and gas wells in the earth, and more particuarly to such a bit having a failure indicator feature.
Rotary drill bits of the roller cutter type, such as shown for example in U.S. Pat. No. 3,923,348, are the most commonly used type of drill bits in the oil and gas well drilling industry because they offer satisfactory rates of penetration and useful lives in drilling most commonly encountered formations. Roller cutter drill bits comprise a bit body having a threaded pin at its upper end adapted to be detachably secured to a drill string suspended from a drill rig, and a plurality of depending legs, typically three such legs, at its lower end. The drill bit further comprises a plurality of conical roller cutters having cutting elements thereon, one for each leg, and bearing means for rotatably mounting each roller cutter on its respective leg. Sealed bearing type roller cutter bits further have a lubrication system comprising a reservoir holding a supply of lubricant, passaging in the bit body extending from the reservoir to the bearing means for flow of lubricant to the bearing means, a seal member between the roller cutter and the bearing journal for holding lubricant in the bit and a diaphragm at the reservoir for providing pressure compensation between the lubricant and the drilling fluid in the annulus between the bit and the well bore.
In use, roller cutter drill bits are rotated in the well bore on the end of a drill string which applies a relatively high downward force thereto. As the bits are rotated, the conical roller cutters rotate on the bearing journals, thereby bringing the cutting elements into engagement with the formation at the bottom of the well bore. The cutting elements drill the formation at the well bore bottom by applying high point loads to the formation for causing it to crack or fracture in compression, and may also drill by scraping or dragging across the formation.
For most cost effective drilling, a worn drill bit should be replaced when the increased cost due to the worn bit's reduced rates of drilling penetration as compared to a new bit (i.e., the cost of increased drilling time, which may be $500 per hour and more) becomes equal to the cost of replacing the bit (i.e., the cost of the new bit plus the cost of rig time in tripping the drill string in and out of the well bore). Unfortunately, once a drill bit is positioned in a well bore, it becomes difficult to gather reliable information regarding its operating condition, its performance and its remaining useful life. Typically, the decision by a rig operator to replace a drill bit is a subjective one. He relies on his experience and in some instances also so-called "offset" data showing the performance of similar bits in drilling similar formations to decide when a bit should be replaced. However, because of the many factors affecting drilling performance besides the condition and performance of the bit itself, the rig operator's decision as to when to replace a bit is often not the most cost effective. Even more serious, the rig operator may unknowingly run the bit so long that it fails, perhaps thereby causing the costly problems described hereinafter. Bit failure may also result from an improper application of the bit, such as by excessive weight on the bit, excessive rotational speed and drilling in the wrong kind (i.e., hardness) of formation, as well as a defect in the bit itself.
Bit failures are typically of two modes namely (1) breakage of the cutting elements and (2) bearing failure. While the first mode is more common, the second may be more serious. In the first mode, pieces of the cutting elements which are either steel teeth or tungsten carbide inserts are broken from the roller cutters. This breakage significantly reduces the rate of drilling penetration, but the broken pieces are typically carried away from the well bore bottom by the circulating drilling fluid thereby leaving the well bore bottom clean for a replacement bit. In contrast, in the second mode, if the bit is continued to be used with a failed bearing assembly, the assembly will no longer be able to hold the roller cutter on the bearing journal and the roller cutter will fall from the bit when the drill string is pulled from the well bore. A lost roller cutter can be retrieved from the well bore bottom only by a time-consuming (and hence expensive) "fishing" operation, in which a special retrieval tool is tripped in and out of the well bore. In sealed bearing roller cutter bits, bearing failure is often the result of a seal failure which allows lubricant to flow out of the bit and drilling fluid having abrasive particles entrained therein to flow in. Although less common, diaphragm failure has the same result as seal failure. In any event, bearing failure is almost always preceded by or at least accompanied by a loss of lubricant.
Numerous bearing failure indicator systems have been proposed for inclusion in drill bits so as to give the rig operator a signal indicating bearing failure. One such system, shown for example in U.S. Pat. Nos. 3,581,564, 3,703,096 and 3,782,190, involves the measurement and interpretation of certain drilling parameters at the drill rig, such as drill string torque, weight on bit, and rate of penetration, to signal drill bit bearing failure. In practice this system has proved to be unreliable. This is likely due to the large number of variables, such as the type of formation and the pressure and flow rate of the drilling fluid, other than bit performance per se which affect the drilling parameters measured at the drill rig.
A second system, such as that shown for example in U.S. Pat. Nos. 3,011,566, 3,678,883 and 3,865,736, involves a marker fluid, such as a radioactive material in the bearing lubricant, which is released into the drilling fluid upon bearing failure. This marker fluid is to be detected at the drill rig when the drilling fluid is circulated back up to the drill rig. While as indicated above, the loss of lubricant in a sealed bearing drill bit precedes or at least accompanies the failure of the bearings of the bit, and thus is a reliable indication of bit failure, this system proved to be unsatisfactory. In addition to the difficulties present in handling radioactive materials, the marker fluid when released into the relatively large volume of drilling fluid was found to become so diluted as to be undetectable.